Anyone know what size they will be. I assume they will be round head.
You say your existing plate is scratched, implying you have at least a plate in your possession. What diameter are the holes in it that are used to secure it to the machine? Find where the plate goes on the cabinet and measure the diameter of the holes already there.
Can you look at some pictures on the lathes.co.uk site to determine the head style? What other styles apart from round head are available on the market?
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With respect to the tailstock, read the instructions for setting it over (as if cutting a taper). Attempt to do that to the maximum extent in both directions. If it does not move, it is seized.